I. Field
The following description relates generally to wireless communications, and more particularly to an approach for simultaneously operating and dynamically scheduling single-user/multi-user multiple-input multiple-output modes.
II. Background
In wireless communications, bandwidth and base station transmit power are regulated. Design around these fixed conditions has led to multiple-input multiple-output (MIMO) systems as a path toward realizing increased peak data rate, spectral efficiency, and quality of service. A MIMO system consists of transmitter(s) and receiver(s) equipped, respectively, with multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NV independent channels, which are also referred to as spatial channels, where NV≦min{NT,NR} Each of the NV independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g. higher throughput, greater capacity, or improved reliability, or any combination thereof) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized. MIMO systems can be divided in two operational classes: (i) Single-user MIMO, and (ii) multi-user MIMO. A main goal of single-user MIMO (SU-MIMO) operation can be to increase peak data rate per terminal, whereas a main goal in multi-user MIMO (MU-MIMO) can be to increase sector (or service cell) capacity. Operation in each of these classes has advantages. SU-MIMO exploits spatial multiplexing to provide increased throughput and reliability, MU-MIMO exploits multi-user multiplexing (or multi-user diversity) to further gains in capacity. Additionally, MU-MIMO benefits from spatial multiplexing even when user equipment has a single receiver antenna.
Reliability, throughput and capacity gains in SU-MIMO and MU-MIMO depend on available channel state information at the transmitter (CSIT), or channel quality information (CQI), used by a base station scheduler. In a SU-MIMO system, CSIT can be obtained under the assumptions of rank adaptation; successive inter-stream interference cancellation (SIC), if the receiver is capable of performing such cancellations; and no inter-user interference (or other-user interference). On the other hand, CSIT in a MU-MIMO system assumes inter-user interference and absence of SIC and rank adaptation. When a base station services simultaneously single-user MIMO and multi-user MIMO terminals, such a mismatch in the nature of CQI leads to interpretations problems at the scheduler in the access point, which in turn degrades performance. In order to mitigate the CQI mismatch problem, base station(s) can separate user equipment (UE) operating in SU-MIMO mode from UE that operates in MU-MIMO; however, such separation decreases multi-user diversity with the ensuing degradation in performance (e.g. throughput, capacity). Alternatively, terminals may report two sets of CQI, one for each operation mode (SU-MIMO or MU-MIMO), but such alternative would result in excessive feedback overhead and related poor performance.
There is therefore a need for CSIT feedback that minimizes degradation of performance in SU-MIMO and MU-MIMO users operating simultaneously in a cell.